The present invention relates to a gas turbine system for producing power. More specifically, the present invention relates to a thermochemically recuperated and steam cooled gas turbine system.
A gas turbine is comprised of a compressor section that produces compressed air that is subsequently heated by burning fuel in a combustion section. The hot gas from the combustion section is directed to a turbine section where the hot gas is used to drive a rotor shaft to produce power.
The turbine section typically employs a plurality of stationary vanes circumferentially arranged in rows. Since such vanes are exposed to the hot gas discharging from the combustion section, cooling these vanes is of utmost importance. Traditionally, cooling was accomplished by bleeding compressed air produced in the compressor section and flowing it through a central passage formed in the airfoil portion of the vane, which is essentially hollow. Typically, a number of small passages were formed inside the vane airfoil that extend from the central passage to the surfaces of the vane, such as the leading and trailing edges or the suction and pressure surfaces. After the cooling air exits the vane passages, it enters and mixes with the hot gas flowing through the turbine section.
Unfortunately, the tradition approach to cooling the turbine vanes has a detrimental impact on the thermal efficiency of the gas turbine. Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process the work recovered from the expansion of the compressed cooling air is much less than that recovered from the expansion of the compressed air heated in the combustors. In fact, as a result of losses due to pressure drop and mechanical efficiency, the work recovered from the cooling air is less than that required to compress the air in the compressor. Thus, it would be desirable to provide cooling to the stationary vanes without the need to bleed compressed air.
Natural gas is one of the most common fuels burned in the combustion section. The major component of natural gas is typically methane. It has been found that at elevated temperature and in the presence of a catalyst, typically nickel based, methane reacts with steam and is converted to hydrogen and carbon monoxide. It has been proposed that such reformed fuel can be advantageously produced and burned in the combustor of a gas turbine by using the exhaust gas discharging from the turbine to heat the natural gas--see, for example, U.S. Pat. No. 5,133,180 (Horner et al.)--thereby increasing the efficiency of the gas turbine and reducing NOx production.
Unfortunately, in order to utilize the turbine exhaust gas as the source of heat for the reforming process, it had been thought that the temperature of the exhaust gas must be in excess of 650.degree. C. (1200.degree. F.), and optimally 815.degree. C. (1500.degree. F.). However, the temperature of the gas exhausting from the turbine section of a gas turbine is typically less than 540.degree. C. (1000.degree. F.). Thus, it has been proposed to use a turbine with high and low pressure sections, with a reheat combustor located between the sections. The reheat combustor could then heat the gas entering the low pressure turbine so that its exhaust was in the appropriate temperature range for reforming the natural gas. Unfortunately, low temperature turbines are generally not capable of such high temperature operation. Alternatively, it has been proposed that a duct burner be incorporated upstream of the reformer to heat the exhaust gas into the required temperature range. However, the additional fuel burned in the duct burner has a negative impact on the thermodynamic efficiency of the gas turbine system.
It is therefore desirable to provide an apparatus and method for heating a mixture of steam and gaseous hydrocarbon fuel in a reformer that utilizes turbine exhaust gas for heating without the need to raise the temperature of the exhaust gas by burning additional fuel.